Directional coupler



Allg- 9, 1949 E. M. PURCELL. 2,478,317

DIRECTIONAL COUPLER I Filed July 9, 1945 FI G` I MI//v now/6 60/05 I@ X jloj INVENTOR EDWARD M. PURCELL 9. ALU.

Y ATTORNEY Patented ug. 9, 1949 zarsen DIRECTIONAL COUPLER Edward M.v Purcell, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application July 9, 1945, Serial No. 604,011

4 Claims. l

This invention relates to an electrical apparatus and more particularly to an improved microwave energy directional coupler.

A description of a one-hole type of directional coupler has been given in an application of Hans A. Bethe entitled Electrical apparatus, filed April 24, 1945, and bearing Serial No. 590,047. The directional coupler there shown includes two parallel lengths of wave guide which are interconnected and coupled by an aperture in a common wall. The common wall and the walls parallel thereto are perpendicular to the direction of the electric elds existing within the wave guides; such Walls are generally the wider Walls of the wave guides and are hereinafter so considered.

The directional coupler there shown functions in such manner that a primary wave traveling in a transmission line including one length of coupler wave guide, hereinafter termed the main guide, produces secondary waves in the other length of coupler wave guide, hereinafter termed the auxiliary guide. Further, the secondary wave energy excited in one s-ection f the length of guide extending in one direction from the coupling aperture) of the auxiliary guide is negligibly small as compared to that excited in the other section (the length extending in. the other direction from the coupling aperture). This condition of high selectivity or directivity is achieved by suitable cross-sectional dimensioning of the component wave guides so that the wider walls have certain critical dimensions. In the instance wher-e it is desired that the wider walls of the main and auxiliary guides be equal, theoretically complete directivity is obtained when the Width of the said walls is 0.707 of the free-space wave length of the microwave energy involved.

Special sizes of wave guides must therefore be utilized for the directional coupler described above, for directional couplers so dimensioned are rarely of the same cross-sectional size as the transmission line with which they are to be used. It then also becomes necessary to provide impedance matching means between the directional coupler and the transmission lin-e for satisfactory performance.

It is an object of the present invention to provide a directional coupler Whose main guide may have the same cross-sectional dimensions as the transmission line to which the directional coupler is to be connected.

It is another object to provide a directional coupler in which high directivity is achieved as a result of a suitable angular relation between the component Wave guides.

It is still another object of the invention to provide a directional coupler which may be adjusted to have high directivity at any frequency within a selected microwave band.

Several advantages are secured by therpresent A wall.

invention. The structure of the directional coupler hereinafter described is mechanically simple, and its Wave guide components may generally be made from the same stock wave guide utilized for the transmission line of a microwave system. Matching problems are eliminated and special matching structures become unnecessary. High directivity may be achieved Without the necessity of providing specially dimensioned directional coupler components. Furthermore, high directivity may be obtained at any microwave frequency Within a Working band, by a corresponding angular adjustment of the directional coupler components.

These and other objects and advantages will become apparent from the following description when read in connection with the drawing, in which:

Fig. 1 is; a schematic diagram representing a top view of component wave guides of a directional coupler in accordance with the present invention;

Fig. 2 is a schematic diagram representing a longitudinal section of Fig. 1; and

Fig. 3 is a cut-away isometric view of a directional coupler embodying the invention.

Similar characters of reference are used in al1 of the above figures to indicate corresponding parts.

Referring now to Figs. 1 and 2 of the drawing, reference numerals I0 and Il designate main and auxiliary guides, respectively each having a rectangular cross section. Electric lines of force exist Within the wave guides during excitation, and in the embodiment here shown the wave guides are so proportioned that the Walls perpendicular to these lines of force are the Wider walls of the waveguides.

The main and vauxiliary guides are assembled with a'wider wall of one in contact with a wider Wall of the other, thus forming a partition or wall which is vcommon to the main and auxiliary guides. The common wall may alternatively be Yformed of a single sheet of suitably thin material as later described. An aperture i3 through the common wall provides electromagnetic coupling between the main and auxiliary guides, whereby Wave energy in the main guide induces or eX- cites wave energy in the auxiliary guide.

The component wave guides are further adapted to be angularly displaced, with respect to each other, about an axis passing through the center of aperture I3 and perpendicular to the common The angle V0 between the longitudinal axes of the wave guides, as seen in the plan view given in Fig. 1, may thus be adjusted to a suitable value for high directivity at the operating frequency as will hereinafter appear. g

As a matter of general practice it is the incident primary wave from which coupling is de- 30 degrees.

sired, and such a wave is shown traveling from right toleftinwaveguide llas indicated by the full-line arrow' |2.in Figs. 1 and 2.0i the drawing. The corresponding reflected primary wave is not here shown, but it is to be understood that as is usual in practical microwave systems its magnitude is small as compared to the incidentprimary wave, and that it travels inthe opposite direction in Wave guide l0.

For a given wave length.oienergyinvolvem an angle may be selected which provides high directivity in the described structure. At such an angle 0, the incident primary wave I2 in main guide I0 excites a secondary wave I4, of proportional magnitude, irrene section of the auxiliaryguide as shown. 'The-path,which may be traced along the direction or travel of the primary wave, thence rthrough'the coupling aperture and along the direction of travel ofthe secondary wave, includes an acute angle as maybe noted by reference to Fig. `1. yThe 'direction of -travel of the secondary wavewith respect to the corresponding primary wave is therefore here defined as backwardly directed, and the section of auxiliary guide in which the secondary wave is excited is similarly dened asbackwardly-extending,rhereinafter also -termed the wave-propagating section.

The component waveguides fmay be Vangularly displaced through lsuchen-angler? that substantionally all of the secondary'energytravels backwardly with respecttosthe primary wave 'from which it is excited, very little energy beingexcited in the forwardly extending'section of the auxiliary guide. The optimum `value of 0 at which such high directivity is secured is determined by the operating frequency, the thickness Yof the common wallffan'dfthe size and shape of the coupling aperture. In "general, the optimum value of `angle 0 increasesas the thickness of the common wall increases, and decreases as the wave length of the energy involved increases. The optimum value of Yangle I0 also `decreases as the size of the coupling aperture increases; the size of the aperture itself is-primarilyv determined by -the amount of coupling desired, and varies directly with it. lIt vhas'been 'found .that'for rectangular waveguides having '-outsi'de dimensions of 1/2 inch by 1A, inch with-a-wall thickness of .04

inch the optimum `values oft l-vary between the 'values of 75 degrees and'f55 degreesas the diameter of aperture I3varies'fromalfmillimeter to ,4 millimeters. These valuesoffare fora thickness of the common wallof 104 .-inch. :If the thickness of the common wall is decreased'to '.005 inch the y range of valueso'f or :thesame rangeof diameters becomes 49 degrees-to 42 degrees being larger for the smaller diameters. For-still other values of wallthicknesa-.aperturesize, etc.the yvalue of 0 may vary from-90 ydegrees to less than The optimum valueof 0 for-,a particular set of conditions is best-determinedex perimentally, usingl-the-above-values asa guide.

The high directivity `obtained in the directional coupler here described is aresult of interaction.

of the electric and magnetic-held,components at :the coupling aperture. The relative :strength of `these eld components, vwhich rinterferezconstructivelyV and destructivelyin:theabackward and forward directions, respectively,.alongthe auxiliary guide, is `determinedlby thef angle vi0 between the longitudinal vaxes ofthe component-Wave guides. Theoretically, an optimum value ofthe angle 0 causes elimination of a 'forwardly directed secondary wave. Asarpractical matter. howeverfa Yrected secondary wave. -e'necessary for high directivity is best obtained from aseries of graphs which may be empirically vthe directional coupler is to operate.

'4 very small secondary wave may nevertheless exist ,inV a, forwardly extending section, but may be'regarded ashaving `substantially zero value as compared to the corresponding backwardly di- The optimum value of .in the transmissionv line of a-microwave system. 'The manner of operation of the disclosed structure is suchA that the -proportion of energy coupled -into the auxiliary guide'remains constant and independent of the location along the transmission line at which main, guide ID is connected.

'The particular'valueof thestanding wave ratio, :and the relative phase of the standing wave with respect to the Acoupling aperture, have no eiect upon the proper operation of the directional coupler as outlined above.

'In the structure shown schematically in Figs. 1rand 2,- the wider wallsof the mainand auxiliary guides may have various dimensions, correspondingly aiecting the optimum value of angle 0. The wider walls ofthe component wave guides `may,'i`or example, be of equal width and greater than 0;'70'7 of the free-space wave length at which If desired, the wider Walls of the main and auxiliary guides may have unequal widths, both being greater, or vone being less and the other greater than the preceding critical dimension. In any one of these instances, however, an angle 0 may be selected for which high directivity is achieved in the directional coupler. It is also evident that the main `guide may alwaysbe made of the same stock .wave guide utllized'for transmission lines.

Referring now tolig. 3 of the drawing, an embodiment of the invention is shown in which theangle 0 is fixed, and which is adapted to communicate a small fraction of either incidentI or reiiected energy in a transmission line to a utilization apparatus, for example to a frequency meter'or other measuringinstrument, The ref- -erence numeral lli-again designates a length of vwave guide which is adapted to be connected in a transmission line of a microwave system. Main guide i@ is here shown provided with flanges l5 and l5 so that it may be joined to a transmission line provided with similar anges. Other means off'providing continuity between a transmission 'line and a directional coupler may be utilized,

however, such as the choke-ange joints shown in the previously mentioned copending application. Associated with main guide I0 is a length of auxiliary guide l'l, vhaving closed ends in the illustrated embodiment. A wider wall of auxiliary guide Il is suitably apertured as indicated at I3, and this wall forms a partition or common wall between the associated 'wave rguides as will be seen. A wider wall of main guide Il] is mortised toreceive the apertured Wall of auxiliary guide Il at an optimum angle 0, and the two lengths of vwave-guide are thus adapted--tofbe intertted and secured together as by soldering or other means.

One section of auxiliary guide II, hereinafter termed the wave-absorbing section, is loaded by a matched termination such as a tapered resistance strip shown at I'I, which functions to absorb wave energy propagated toward it. Reections which would ordinarily otherwise occur are thereby avoided, so that the desired backwardly directed energy in the other section (wave propagating section) of auxiliary guide I I is not interfered with. A pick-up probe I8 extends into the wave propagating section from a coaxial connector I9, located at a suitable distance from the corresponding end wall, and provides proper matching between the wave propagating section and a coaxial cable, not shown.

The directional coupler structure thus described functions in the manner previously set forth. A primary wave traveling through main guide I0, in the direction indicated by arrow in Fig. 3, excites a proportionately smaller secondary wave in the backwardly extending section of auxiliary guide Il. Energy from the secondary Wave is communicated to a utilization apparatus by means of an interconnecting coaxial cable, not shown, which is adapted to be coupled to the backwardly extending or wave-propagating section by means of coaxial connector I9 and pick-up probe I8. Any wave energy which may be excited in the wave absorbing section of the auxiliary guide Il I, for example by an oppositely directed primary wave, is prevented by absorbing means I1 from producing disturbing reiiections which would interfere with wave energy in the wave propagating section.

The structure shown in Fig. 3 may be modiiied to have a common wall whose thickness differs from that of the wave guide walls. For example, both the main and auxiliary guides may be mortised and ltted together on opposite sides of an apertured sheet of metal of suitable rhomboidal shape and size, which then functions as the common wall. Electrical continuity may here again be secured as by soldering or other means.

The wave selectivity achieved in the directional coupler of the subject invention is substantially independent of the magnitude and relative phase of standing waves in the transmission line with which it is used. The directional coupler may, therefore, be readily used for measurement of standing wave ratios, for separate measurement of component incident and reilected waves, and in general to provide attenuated coupling to components of a micro-wave system.

It is apparent that the invention is not limited to the physical construction illustrated in the drawing, but that various changes may be made without departing from the spirit of the invention.

What is claimed is:

1. A microwave energy coupler, including a pair of rectangular waveguides interconnected by a common broad wall, said broad wall being formed with a coupling aperture therein, said pair of waveguides being relatively angularly displaced about an axis through said aperture and perpendicular to said common wall at an angle dependent upon the frequency of the microwave energy, the thickness of said common wall, the size and shape of the coupling aperture and said waveguides dimensions, whereby high directivity is secured.

2. A microwave energy coupler, including a pair of rectangular waveguides interconnected by a common broad wall, said Abroad wall being formed with a coupling aperture therein, said pair of waveguides being relatively displaced by an angle that is more than zero and less than degrees about an axis through said aperture and perpendicular to said common wall, said angle being determined by the frequency of the microwave energy, the thickness of said common wall the size and shape of the coupling aperture and said waveguides dimensions, whereby high directivity is secured.

3. In a system for directional coupling of microwave energy including a first rectangular waveguide having microwave energy propagated through it in a forward direction, a second rectangular waveguide, said waveguides having their broader walls parallel and contiguous, said walls having aligned apertures therein for coupling energy between said waveguides, said waveguides being rotatable with respect to each other about an axis which passes through said coupling apertures perpendicular to said broader walls and rotatable through an angle between zero and ninety degrees whereby at one position of rotation the energy traveling in a backward direction through said second waveguide is at a maximum and energy traveling in a forward direction in said second waveguide i-s at a minimum.

4. A microwave energy directional coupler comprising a first rectangular waveguide having an input end and an output end, a second rectangular waveguide positioned at an acute angle with respect to said rst waveguide, said rst and second waveguides having their broader walls in parallel planes and juxtaposed to form a common wall where they cross, a coupling aperture in said common wall at the vertex of said angle, means to derive an output located in the end of r second waveguide which is nearest the input end of said rst waveguide, and means to absorb energy located in the end of said second Waveguide which is nearest the output end of said rst waveguide.

EDWARD M. PURCELL.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 2,153,728 Southworth Apr. 11, 1939v 2,241,119 Dallenbach May 6, 1941 2,407,318 Mieher Sept. 10, 1946 2,423,390 Korman July 1, 1947 2,427,106 Landon Sept, 9, 1947 FOREIGN PATENTS Number Country Date 545,936 Great Britain June 18, 1942 

